Passive Transport

Core Explanation

Passive transport is the movement of substances across a membrane without the input of metabolic energy (ATP). It is driven by the kinetic energy of molecules and occurs spontaneously down a concentration or electrochemical gradient. Passive transport includes three distinct mechanisms: simple diffusion, facilitated diffusion, and osmosis.

Simple Diffusion

Diffusion is the net movement of molecules from a region of higher concentration to a region of lower concentration, resulting from random thermal motion. In the context of membranes:

• Molecules that can cross the lipid bilayer directly (small nonpolar molecules: O₂, CO₂, N₂; small polar molecules like water and ethanol to a limited degree) diffuse down their concentration gradient.
• No membrane proteins are involved.
• No energy (ATP) is consumed.
• Net movement continues until equilibrium is reached, at which point molecules continue to cross the membrane in both directions at equal rates — there is no net movement, but movement does not stop.

Facilitated Diffusion

Many biologically important molecules — glucose, amino acids, ions, nucleotides — are too large or too polar to cross the lipid bilayer on their own. Facilitated diffusion provides a pathway through the membrane via transport proteins, still without consuming ATP. There are two major classes of transport proteins:

Channel Proteins

Channel proteins form hydrophilic pores through the membrane:

• Ion channels are highly selective for specific ions (Na⁺, K⁺, Ca²⁺, Cl⁻) based on pore size and the arrangement of amino acid residues lining the pore.
• Aquaporins are water-specific channels that dramatically increase the rate of water movement across membranes — critical in kidney tubules, plant roots, and other tissues where rapid water flux is needed.
• Many ion channels are gated — they open or close in response to specific stimuli:
  • Voltage-gated: Open/close in response to changes in membrane potential (e.g., Na⁺ and K⁺ channels in neurons)
  • Ligand-gated: Open when a specific molecule (ligand) binds (e.g., neurotransmitter-gated ion channels at synapses)
  • Mechanically-gated: Open in response to mechanical forces (e.g., stretch, pressure, sound waves in the inner ear)
• Channels are generally faster than carriers because ions flow through an open pore rather than requiring a conformational change.

Carrier Proteins

Carrier proteins (also called transporters or permeases) undergo conformational changes to move solutes across the membrane:

• The solute binds to a specific site on the carrier.
• The carrier changes shape, translocating the solute across the membrane.
• The carrier releases the solute on the other side and returns to its original conformation.
• Carriers are slower than channels (typically hundreds to thousands of molecules per second vs millions for channels) but can be highly specific.
• An important property of carriers is saturation: at high solute concentrations, all carrier binding sites are occupied, and the transport rate reaches a maximum (Vmax). This is analogous to enzyme kinetics.

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane from a region of lower solute concentration (higher water concentration) to a region of higher solute concentration (lower water concentration). In other words, water moves toward the side with more dissolved particles — effectively "trying" to equalize solute concentrations by diluting the more concentrated side.

The direction of water movement is described by tonicity — the relative solute concentration of a solution compared to the inside of a cell:

Plant cells have an advantage in hypotonic environments: the rigid cell wall resists expansion, generating turgor pressure that keeps the plant upright. When plant cells lose turgor (in hypertonic conditions or drought), the plant wilts. Animal cells, lacking cell walls, are vulnerable to osmotic lysis in hypotonic environments — which is why IV fluids must be isotonic.

How It Works

The Energetics of Passive Transport

Passive transport is thermodynamically favorable because it increases entropy (disorder). When solute molecules spread out from a concentrated region, the number of possible molecular arrangements increases — entropy increases. This is the same reason a drop of dye spreads evenly throughout a glass of water. No energy input is needed; the process is spontaneous.

The free energy change for moving a mole of uncharged solute from concentration C₁ to C₂ is:

ΔG = RT ln(C₂/C₁) — for uncharged solutes

For movement down a gradient (C₂ < C₁), ΔG is negative (spontaneous). For movement up a gradient (C₂ > C₁), ΔG is positive — which is why active transport requires ATP.

Why Transport Proteins Are Needed

The hydrophobic core of the lipid bilayer blocks the passage of polar and charged molecules. Transport proteins solve this problem by providing a hydrophilic pathway through the membrane:

• Channels create an aqueous pore.
• Carriers shield the solute from the hydrophobic interior during the conformational change.

Both mechanisms make transport specific — only molecules that fit the binding site or pore can pass.

Compare and Contrast

Biological / Medical Relevance

• IV fluids: Normal saline (0.9% NaCl) and lactated Ringer's solution are isotonic to prevent red blood cell damage
• Cerebral edema: Hypertonic solutions (e.g., mannitol) are administered to draw water out of swollen brain tissue
• Diuretics: Many act by altering ion transport in kidney tubules, changing the osmotic gradient and affecting water reabsorption
• Cholera: The cholera toxin opens intestinal chloride channels, causing massive ion and water loss into the gut lumen → severe dehydration
• Cystic fibrosis: The CFTR chloride channel is defective, disrupting water balance across epithelial surfaces → thick mucus
• Dialysis: Artificial kidneys use concentration gradients across semipermeable membranes to remove waste products from blood

Common Misconceptions and Exam Traps

• Misconception: Diffusion stops at equilibrium. Reality: Individual molecules continue to move; net movement stops, but exchange continues in both directions.
• Exam trap: Confusing "concentration" with "tonicity." Tonicity depends on the concentration of non-penetrating solutes — solutes that cannot cross the membrane. Urea, for example, can cross most cell membranes, so a urea solution may not have the tonicity its total concentration suggests.
• Misconception: Water moves toward higher water concentration. Reality: Water moves toward higher solute concentration (lower water concentration). These are equivalent statements, but the "toward solutes" framing is often easier to work with.
• Exam trap: Thinking that facilitated diffusion requires energy because it uses a protein. The transport protein provides a pathway, not energy. No ATP is consumed.
• Misconception: Plant cells in isotonic solutions are "healthy." Reality: Plant cells are healthiest in hypotonic environments where turgor pressure supports the plant. In isotonic conditions, plant cells are flaccid and the plant wilts.